Add atbash-cipher decode tests

[verdammelt]

These tests were never implemented (but were listed in the tests.toml
file as if they had been).

Fixes #871

[github-actions]

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[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

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[no important files changed]

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[verdammelt]

@BNAndras I found it rather simple (and a nice distraction) to implement these missing tests. Could you give them a quick review? Thanks.